Time-Dependent Correlation of the Microbial Community and the Metabolomics of Traditional Barley Nuruk Starter Fermentation

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The microbial community and the metabolites of barley nuruk were studied to determine the time-dependent correlation between the fermentation of microbes and metabolites. Samples were analyzed by a polyphasic approach based on culture-dependent, culture-independent (PCR-DGGE and qPCR analysis), and metabolite analysis using GC-MS. Barley nuruk consists of varying amounts of bacteria, yeasts, and molds. The PCR-DGGE results showed that only one phylotype, Aspergillus oryzae, was predominant throughout fermentation, reaching a maximum on day 9. The bacterial load was higher on day 6 of fermentation, and then gradually decreased because of increased fungal activity. The shift in fungal and bacterial diversity observed by DGGE was further confirmed by qPCR analysis. In addition, microbes closely related to Pantoea agglomerans and Saccharomycopsis fibuligera appeared to play key roles in the fermentation of barley nuruk. GC-MS analysis combined with multivariate analysis, including PCA, PLS-DA, and OPLS-DA, showed fermentation time-dependent metabolite patterns. A total of 21 metabolites, including organic acids, amino acids, sugars, and sugar alcohols, were identified. In particular, glycerol, malic acid, fructose, glucose, sucrose, and maltose were produced at the early fermentation stages (0–6 d), whereas glutamine, aspartic acid, glutamic acid, mannitol, and xylitol were produced during the latter stages of fermentation (9–18 d). Mixed culture fermentation was found throughout the natural fermentation of barley nuruk starter. Most likely, A. oryzae had a major role in saccharification, along with other mixed cultures.